Devices for purifying anaerobic biological wastewater are well known. Usually these consist of a single reactor chamber, into whose bottom area flows the wastewater, which is to be clarified. A sludge bed, which represents the biologically active zone for purifying the wastewater and through which the wastewater enters, is provided in a lower area of the reactor chamber. The wastewater is purified while flowing through the sludge bed, its organic content is reduced and biogas (essentially consisting of methane and carbon dioxide) is built-up. The built-up biogas rises through the sludge bed and the zone lying above it, is trapped by separator baffles and discharged to a gas outlet pipe. Above the separator baffles is a calming zone, whose purpose is to enable entrained biologically activated sludge particles to fall out and settle again. The purified wastewater is removed from the upper end of the calming zone.
Problems arise with this arrangement inter alia due to the accumulation of floating sludge below the separators. Also it is necessary to filter the exhausted air due to gases being released in the calming zone, since otherwise serious odour problems would arise. Furthermore uneven flows in the sludge bed can arise due to insufficient thorough mixing of the sludge bed, as a result of which the purifying efficiency of the plant is reduced.
Devices of the type initially specified are known from EP 0 170 332 A1 and EP 0 539 430 B1. Lifting pipes emerging from an area lying above the feed pipe for the wastewater (and above the sludge bed) and below the fluid level in the main chamber, to which rising gas trapped by separator baffles is fed, are provided here. Gas and wastewater, which has been purified but still contains sludge particles, are lifted through the at least one lifting pipe to a return tank by the pressure of the gas. Biogas and the entrained wastewater are separated in the return tank. The biogas is diverted to a gas outlet pipe and the wastewater together with the contained sludge particles flows back from the return tank through a return pipe to the main chamber, and to be precise to the bottom area of the same, where thorough mixing in the sludge bed takes place. Above the lifting pipes with the associated separator baffles is a calming zone and further separator baffles may be provided there if necessary. The purified wastewater flows out from the upper area of the main chamber. Also problems arise with this arrangement inter alia due to the accumulation of floating sludge in the separators and in the wastewater removal area, which entails high maintenance costs.
A distinction is to be made between devices for purifying wastewater and such for digesting sludge with a high organic content, which is biologically degradable, as occurring in agriculture or industry. Such sludge before treatment has 10-20% dry substance (after treatment correspondingly less, 5-6% for example). In the case of wastewater on the other hand the dry substance content is below 1%, normally in the range of 0.1%. Usually the devices for digesting sludge are not suitable for wastewater purification. Devices for digesting sludge are known for example from DE 32 11 888 A1, DE 82 11 869 U1 and U.S. Pat. No. 4,302,329 A.
The device known from DE 33 30 696 A1 is applicable both for digesting sludge and for purifying anaerobic wastewater. The main chamber, in which the main organic activity takes place, has lateral overflow outlets, which lead to a separating chamber surrounding the main chamber. On the outer edge of the separating chamber is an overflow channel, from which the discharge pipe for the treated substrate emerges. A current is created first downwards and then upwards by a partition wall, which is open at the bottom in the separating chamber. Organically activated sludge, which can be pumped back through a return pipe to the main chamber by means of a pump, settles at the bottom of the separating chamber. Also in the case of this device floating sludge accumulates in the main chamber and inside the separating chamber, as a result of which maintenance work is necessary. Furthermore retention of biologically activated sludge is not optimum. If an excessive amount of biologically activated sludge is shed, the biological activity in the main chamber however is reduced to an unacceptable degree.